TWEAKING gravity cannot explain why NASA’s twin Pioneer spaceships are drifting off course as they fly across the solar system.
Launched in the early 1970s, the Pioneer 10 and 11 probes are approaching the edge of the solar system from opposite directions, gradually slowing down as the sun’s gravity tugs at them. But they are slowing down slightly more than expected and no one quite knows why. Some physicists argue that stronger gravity in the outer solar system can explain the so-called Pioneer anomaly.
But when Kjell Tangen, a physicist at the firm DNV in Hovik, Norway, modified gravity to…
